The most common gas discharge tube in use today is the neon sign. When a current is passed through an inert gas such as neon or argon held in a discharge tube, the gas will glow at a characteristic color, such as red in the case of neon. In order to excite the gas in a discharge tube, a sufficiently high voltage must be maintained between electrodes on either end of the discharge tube to allow current to flow. This calls for a high voltage power supply to drive the tube.
Excitation power supplies, and in particular neon light transformers of the prior art, have been known for many years. The most common neon light transformer is a 60 Hz, 120 VAC primary with a 60 Hz approximately 10 KV secondary which is directly connected to the electrodes attached to either end of the neon sign. A transformer of this size tends to weight 10-20 pounds due to the massive core, number of primary and secondary windings, and the potting of the transformer in a tar-like material to prevent arcing. This results in a very large, bulky and unsightly excitation supply.
More recently, light-weight switching power supplies have been used to set up the 60 Hz 120 VAC voltage to a higher frequency, higher fixed voltage level for exciting discharge tubes. In general, the switching frequency is fixed at the factory and not matched against the load impedance of the gas discharge tube to which it is attached, resulting in a fixed output voltage. This impedance mismatch causes a great loss in efficiency and sometimes an interesting side effect. The length and volume of the discharge tube as well as the gas pressure, temperature and type of gas used in the discharge tube all have an effect on the characteristic impedance of the discharge tube. A fixed frequency, fixed output impedance excitation supply attached to a variety of gas discharge tubes may cause impedance mismatches which could result in the "bubble effect". This effect is caused by standing waves appearing at a high frequency within the discharge tube, resulting in alternate areas of light and dark in the tube. The standing wave may not be exactly matched to the length of the tube, resulting in a scrolling or crawling bubble effect in which the bubbles slowly move toward one end of the tube. This may be an undesirable effect in some neon signs, or may be desired in others. The problem, however, is that with fixed frequency output gas discharge tube excitation supplies, the resulting effect is unpredictable.
The prior art also developed variable frequency switching power supplies for exciting gas discharge tubes to make the foregoing bubble effect more predictable. By attaching an excitation supply to a gas discharge tube and varying the frequency, one could either eliminate or accentuate the bubble effect. This resulted in an acceptable solution to the unpredictability of the bubble effect, but did not solve the impedance mismatch problem or allow a variable output voltage for setting the optimal brightness. In order to get the best transfer flow of power from the excitation supply through the gas discharge tube, the output impedance of the switching supply must be matched to the input impedance seen at the terminals of the discharge tube. The frequency at which this impedance match is most closely satisfied may actually result in a bubble effect when one is not needed, or may not result in a bubble effect when one is desired. In order to satisfy the user with the correct aesthetic result the frequency must be varied, which may result in an impedance mismatch. An impedance mismatch results in a less than optimal output voltage from the supply and light output of the discharge tube, no excitation at all, standing waves (either fixed or moving, or any combination of the above. Thus, if a user varies the frequency of a variable frequency excitation supply to obtain the desired aesthetic effect of the bubble effect, the resulting unmatched impedance may cause the discharge tube to be too dim or too bright.
There is also a need in the prior art to prevent overvoltage runaway in high voltage power supplies. Allowing a power supply to operate without a load may damage the supply.
Thus, there is a need in the prior art for a variable frequency, variable output voltage excitation supply which allows for matching or varying the output impedance of the transformer to most closely match the input impedance of a variety of gas discharge tubes in order to gain the optimal combination of intensity and bubble effect. There is also a need to prevent overvoltage runaway in such a power supply.